Fluoropolymers are applied to a wide number of substrates in order to confer release, chemical and heat resistance, corrosion protection, cleanability, low flammability, and weatherability. Coatings of polytetrafluoroethylene (PTFE) homopolymers and modified PTFE provide the highest heat stability among the fluoropolymers, but unlike tetrafluoroethylene (TFE) copolymers, cannot be melt processed to form films and coatings. Therefore other processes have been developed for applying coatings of PTFE homopolymers and modified PTFE. One such process is dispersion coating which applies the fluoropolymer in dispersion form. Dispersion coating processes typically employ such fluoropolymer dispersions in a more concentrated form than the as-polymerized dispersion. These concentrated dispersions contain a significant quantity of surfactant, e.g. 6-8 weight percent. Such dispersion coating processes include the steps of applying concentrated dispersion to a substrate by common techniques such as spraying, roller or curtain coating; drying the substrate to remove volatile components; and baking the substrate. When baking temperatures are high enough, the primary dispersion particles fuse and become a coherent mass. Baking at high temperatures to fuse the particles is often referred to as sintering. In many applications, the performance of a fluoropolymer coating is dependent on the thickness of the film applied and a thick coating is frequently desired. However, if fluoropolymer dispersions are applied too thickly in a single application, the coating will suffer crack formation and the quality of the coating will be diminished or rendered unacceptable for the desired use. Consequently, when thicker coatings are desired, a dispersion coating process essentially requires several passes to create a coating of the desired thickness. There is an economic penalty for additional passes and coating processes with fewer passes are highly desirable. In addition, to increase coating thickness per pass, formulations often include large amounts of non-ionic surfactant and polymeric acrylic film-forming aids. High levels of such materials can be detrimental by imparting unwanted color and increasing the amount of carbonaceous residues in the film after sintering. These residues may interfere with the release properties of the film.
The suitability of fluoropolymer dispersions for forming thick coatings can be evaluated using a test referred to as Critical Cracking Thickness (CCT). Critical Cracking Thickness is a measure of the thickness of a coating formed from polymer dispersion that can be applied to a substrate in one pass without cracking during drying and subsequent baking.
Previous attempts to overcome the problem of crack formation are found in the inventions of Blaedel et al, U.S. Pat. No. 5,576,381. Blaedel et al. propose a mixture of dispersions of non-melt-processible fluoropolymers A and B of varying particle size. Fluorine polymer A has a number average particle size of from 180-400 nm and fluorine polymer B has a number average particle size lower by a factor of from about 0.3 to approximately 0.7, the entire dispersion having a non-monomodal number distribution of the particle diameter. In PCT publication WO 9858984, Blaedel et al. propose another mixture of fluoropolymer dispersions of varying particle size. In this invention, fluorine polymer A has a number average particle size of at least 200 nm and fluorine polymer B has a number average particle size of at most 100 nm. One of the components of A and B is a thermoplastic and one is non-melt-processible and the entire dispersion has a non-monomodal number distribution of the particle diameter. Both dispersion mixtures are disclosed as suitable for soaking, impregnating or coating porous surfaces of fibers and fabrics as well as smooth substrates such as metal, ceramic, glass and plastic substrates.
Lenti et al. have also addressed the problem of crack formation in dispersion applied coatings. European Patent Application 0 969 055A1 targets increasing CCT of dispersion coated metal by providing a mixture of copolymer dispersions also of varying particle size. TFE copolymer of dispersion A has an average particle size in the range of 180-400 nm and TFE copolymer of dispersion B has an average particle size of 20-60 nm and the particle sizes of dispersion B compared to those of dispersion A is lower than 0.3. Aqueous dispersions of A are obtainable by standard emulsion polymerization processes where aqueous dispersions of B are preferably obtained by the microemulsion process described in MI 98A001519.
Yet another mixture of dispersions is proposed in European Patent Application 1 059 342A1 to Marchese et al. In this reference increased CCT is attributed to a mixture of dispersions. That mixture is a combination of dispersion A of TFE homopolymer or copolymer particles with smaller particles of dispersion B of TFE copolymer that is not melt-processible. Dispersion B has limitations on the content of fibril-type particles (i.e., particles having a length/diameter of greater than five), with low amounts being preferred in order to avoid a decrease in CCT and other characteristics of coating performance over time.
To date the prior art disclosures for manufacturing coating compositions of high molecular weight fluoropolymer have been expensive and have failed to consistently achieve a level of significant CCT without blending two types of dispersion. What is desired is a non-melt-processible dispersion composition of high molecular weight fluoropolymer with the heat stability and abrasion resistance of high molecular weight PTFE, a high CCT and ease of application onto metal substrates and glass fabric for many uses in the architectural, industrial and home appliance areas. In addition, such fluoropolymer dispersion would be easy to manufacture having properties without variations from batch-to-batch due to blending inconsistencies.
Further the prior art solutions that address dispersion coatings with a high CCT neglect the important aspect of shear stability of the dispersion. In dispersion coating applications such as curtain coating, a fraction of the coating stream is deposited on the substrate requiring the remainder of the stream to be recycled. The recycled fraction needs to be able to withstand the subsequent multiple pumping and mixing operations necessary for a continuous process. A dispersion suitable for such processing should not easily coagulate when subjected to shearing forces. The resistance of the dispersion to premature coagulation can be measured by a parameter known as gel time and is an indication of the shear stability of the dispersion.